1. Field of the Invention
The present invention generally relates to an apparatus for dropping plugs into a wellbore. More particularly, the invention relates to a plug-dropping container for releasing plugs and other objects into a wellbore, such as during cementing operations.
2. Description of the Related Art
In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. After drilling a predetermined depth, the drill string and bit are removed and the wellbore is lined with a string of casing. An annular area is thus formed between the string of casing and the formation. A cementing operation is then conducted in order to fill the annular area with cement. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production of hydrocarbons.
It is common to employ more than one string of casing in a wellbore. In this respect, a first string of casing is set in the wellbore when the well is drilled to a first designated depth. The first string of casing is hung from the surface, and then cement is circulated into the annulus behind the casing. The well is then drilled to a second designated depth, and a second string of casing, or liner, is run into the well. The second string is set at a depth such that the upper portion of the second string of casing overlaps the lower portion of the first string of casing. The second liner string is then fixed or “hung” off of the existing casing. Afterwards, the second casing string is also cemented. This process is typically repeated with additional liner strings until the well has been drilled to total depth. In this manner, wells are typically formed with two or more strings of casing of an ever-decreasing diameter.
In the process of forming a wellbore, it is sometimes desirable to utilize various plugs. Plugs typically define an elongated elastomeric body used to separate fluids pumped into a wellbore. Plugs are commonly used, for example, during the cementing operations for a liner.
The process of cementing a liner into a wellbore typically involves the use of liner wiper plugs and drill-pipe darts. A liner wiper plug is typically located inside the top of a liner, and is lowered into the wellbore with the liner at the bottom of a working string. The liner wiper plug has radial wipers to contact and wipe the inside of the liner as the plug travels down the liner. The liner wiper plug has a cylindrical bore through it to allow passage of fluids.
After a sufficient volume of circulating fluid or cement has been placed into the wellbore, a drill pipe dart or pump-down plug, is deployed. Using drilling mud, cement, or other displacement fluid, the dart is pumped into the working string. As the dart travels downhole, it seats against the liner wiper plug, closing off the internal bore through the liner wiper plug. Hydraulic pressure above the dart forces the dart and the wiper plug to dislodge from the bottom of the working string and to be pumped down the liner together. This forces the circulating fluid or cement that is ahead of the wiper plug and dart to travel down the liner and out into the liner annulus.
Typically, darts used during a cementing operation are held at the surface by plug-dropping containers. The plug-dropping container is incorporated into the cementing head above the wellbore. Fluid is directed to bypass the plug within the container until it is ready for release, at which time the fluid is directed to flow behind the plug and force it downhole. Existing plug-dropping containers, such as cementing heads, utilize a variety of designs for allowing fluid to bypass the plug before it is released. One design used is an externally plumbed bypass connected to the bore body of the container. The external bypass directs the fluid to enter the bore at a point below the plug position. When the plug is ready for release, an external valve is actuated to direct the fluid to enter the bore at a point above the plug, thereby releasing the plug into the wellbore.
Another commonly used design is an internal bypass system having a second bore in the main body of the cementing head. In this design, fluid is directed to flow into the bypass until a plug is ready for release. Thereafter, an internal valve is actuated and the flow is directed on to the plug.
There are disadvantages to both the external and internal bypass plug container systems. Externally plumbed bypasses are bulky because of the external manifold used for directing fluid. Because it is often necessary to rotate or reciprocate the plug container, or cementing head, during operation, it is desirable to maintain a compact plug container without unnecessary projections extending from the bore body. As for the internal bypass, an internal bypass requires costly machining and an internal valve to direct fluid flow. Additionally, the internal valve is subject to erosion by cement and drilling fluid.
In another prior art arrangement, a canister containing a plug is placed inside the bore of the plug container. The canister initially sits on a plunger. Fluid is allowed to bypass the canister and plunger until the plug is ready for release. Upon release from the plunger, the canister is forced downward by gravity and/or fluid flow and lands on a seat. The seat is designed to stop the fluid from flowing around the canister and to redirect the flow in to the canister in order to release the plug. However, this design does not utilize a positive release mechanism wherein the plug is released directly. If the cement and debris is not cleaned out of the bore, downward movement of the canister is impeded. This, in turn, will prevent the canister from landing on the seat so as to close off the bypass. If the bypass is not closed off, the fluid is not redirected through the canister to force the plug into the wellbore. As a result, the plug is retained in the canister even though the canister is “released.”
The release mechanism in some of the container designs described above involves a threaded plunger that extends out from the bore body of the container, and requires many turns to release the plug. The plunger adds to the bulkiness of the container and increases the possibility of damage to the head member of the plug container. Furthermore, cross-holes are machined in the main body for plunger attachment. Because a plug container typically carries a heavy load due to the large amount of tubular joints hanging below it, it is desirable to minimize the size of the cross-holes because of their adverse effect on the tensile strength of the container.
In order to overcome the above obstacles, plug-dropping containers have been developed that allow release of a dart by rotating a cylindrical valve that allows the dart to pass through an internal channel and at the same time redirect the flow path to be through the canister. Known plug dropping containers of this configuration have valve designs that are complex to manufacture and require the flow to traverse a tortuous and often restricting path in the bypass position.
An example of such a plug-dropping container is shown at 100 in the Prior Art view of FIG. 1. The plug-dropping container 100 first comprises a housing 120. The housing 120 defines a tubular body having a top end, a bottom end, and having a fluid channel 122 therebetween. In FIG. 1, the housing 120 is shown disposed within a cementing head 10. The upper end of the housing 120 may be threadedly connected to an upper body portion 20 of the cementing head 10, or may be integral as shown in FIG. 1. This exemplary plug-dropping container of FIG. 1 is shown in FIG. 3 of U.S. Pat. No. 5,890,537 issued to Lavaure, et al. in 1999, and is described more fully therein.
Disposed generally co-axially within the housing 120 is a canister 130. The canister 130 is likewise a tubular shaped member which resides within the housing 120 of the plug-dropping container 100. This means that the outer diameter of the canister 130 is less than the inner diameter of the housing 120. At the same time, the inner diameter of the canister 130 is dimensioned to generally match the inner diameter of fluid flow channel 22 for the cementing head 10. As with the housing 120, the canister 130 has a top opening and a bottom opening. In the arrangement shown in FIG. 1, the top opening of the canister 130 is in fluid communication with the upper fluid flow channel 22. A simple slip fit is typically provided. The canister 130 has a fluid flow channel 132 placed along its longitudinal axis. The fluid flow channels 122, 132 for the housing 120 and for the canister 130, respectively, are co-axial with the fluid flow channel 22 for the cementing head 10.
A dart 80 is shown placed within the canister 130. The dart 80 is retained within the canister 130 by a plug-retaining valve 140 (shown more fully in FIGS. 2A–2B). The purpose of the plug-retaining valve 140 is to allow the drilling operator to selectively release a dart 80 or other plug into the wellbore. To this end, the valve 140 is constructed to have a plug-retained position, and a plug-released position. Fluid circulation is maintained in both positions of the valve 140.
A bypass area 36 is provided above the canister 130. The bypass area 36 permits fluid to be diverted into an annular region 126 around the canister 130 when the valve 140 is in its plug-retained position.
FIG. 2A presents an isometric view of the plug-retaining valve 140 designed to fit into the opening 40 in the plug-dropping container 100 of FIG. 1. FIG. 2B is a longitudinal cross-sectional view of the prior art valve 140 of FIG. 2A, with the view taken across line B—B of FIG. 2A.
The valve 140 defines a short, cylindrical body having walls 144, 144′. The walls 144, 144′ have an essentially circular cross-section. The wall 144′ is configured to inhibit the flow of fluids from the canister 130 when the valve 140 is rotated to its plug-retained position.
Various openings are provided along the walls 144, 144′ of the plug-retaining valve 140. First, one or more bypass openings 148 are placed at ends of the valve 140. FIG. 2A presents a pair of bypass openings 148. The bypass openings 148 are also seen in the FIG. 2B, which is a cross-sectional view of the plug-retaining valve 140 taken across line B—B of FIG. 2A. The bypass openings 148 receive fluid from the housing-canister annulus 122 when the valve 140 is in its plug-retained position. From there, fluid exits the valve 140 into the lower channel 32.
The plug-retaining valve 140 is designed to be rotated about a pivoting connection between plug-retained and plug-released positions. Rotation is preferably accomplished by turning a shaft 47 (shown in FIG. 1).
The plug-retaining device 140 also has a fluid channel 146 fabricated therein. The fluid channel 146 is oriented normal to the longitudinal axis of the valve 140. In addition, the longitudinal axis of the channel 146 is normal to the axis of rotation of the plug-retaining device 100 when rotating between the plug-retained and plug-released positions. The channel 146 is dimensioned to receive the dart 80 when the plug-retaining device 140 is rotated into its plug-released position during a cementing or other fluid circulation operation. The channel 146 is seen in the isometric view of FIG. 2A, as well as in the cross-sectional view of FIG. 2B.
The housing for the plug-retaining valve 140 from the prior art is cumbersome to manufacture. In this respect, the housing for the valve 140 requires extensive machining to form mating bores for openings 148.
Therefore, there is a need for plug-dropping container for a cementing head having an improved plug-retaining mechanism. There is a further need for a. plug-dropping container that is easier and less expensive to manufacture. Still further, there is a need for a plug-dropping container that provides a less restrictive and less tortuous fluid flow path in its plug-retained position.